The present invention relates to signal communications and, in particular, to soft information related to signals received over a communications channel.
One type of communications channel which is expanding particularly rapidly is wireless communications, particularly as more radio spectrum becomes available for commercial use and as cellular phones become more commonplace. In addition, analog wireless communications are gradually being supplemented and even replaced by digital communications. In digital voice communications, speech is typically represented by a series of bits which may be modulated and transmitted from a base station of a cellular communications network to a mobile terminal device such as a cellular phone. The phone may demodulate the received waveform to recover the bits, which are then converted back into speech. In addition to voice communications, there is also a growing demand for data services, such as e-mail and Internet access, which typically utilize digital communications.
There are many types of digital communications systems. Traditionally, frequency-division-multiple-access (FDMA) is used to divide the spectrum up into a plurality of radio channels corresponding to different carrier frequencies. In time division multiple access (TDMA) systems, carriers may be divided into time slots, as is done, for example, in the digital advanced mobile phone service (D-AMPS) and the global system for mobile communication (GSM) standard digital cellular systems. Alternatively, multiple users can use a common range of frequencies using spread-spectrum techniques as is typically done in code-division multiple-access (CDMA).
A typical digital communications system 19 is shown in FIG. 1. Digital symbols are provided to the transmitter 20, which maps the symbols into a representation appropriate for the transmission medium or channel (e.g. radio channel) and couples the signal to the transmission medium via antenna 22. The transmitted signal passes through the channel 24 and is received at the antenna 26. The received signal is passed to the receiver 28. The receiver 28 includes a radio processor 30, a baseband signal processor 32, and a post processing unit 34.
The radio processor 30 typically tunes to the desired band and desired carrier frequency, then amplifies, mixes, and filters the signal to a baseband. At some point the signal may be sampled and quantized, ultimately providing a sequence of baseband received samples. As the original radio signal generally has in-phase (I) and quadrature (Q) components, the baseband samples typically have I and Q components, giving rise to complex, baseband samples.
The baseband processor 32 may be used to detect the digital symbols that were transmitted. It may produce soft information as well, which gives information regarding the likelihood of the detected symbol values. The post processing unit 34 typically performs functions that depend on the particular communications application. For example, it may convert digital symbols into speech using a speech decoder.
A typical transmitter is shown in FIG. 2. Information bits, which may represent speech, images, video, text, or other content material, are provided to forward-error-correction (FEC) encoder 40, which encodes some or all of the information bits using, for example, a convolutional encoder. The FEC encoder 40 produces coded bits, which are provided to an interleaver 42, which reorders the bits to provide interleaved bits. These interleaved bits are provided to a modulator 44, which applies an appropriate modulation for transmission. The interleaver 42 may perform any of a number of types of interleaving. One example is block interleaving, which is illustrated in FIG. 3. Conceptually, bits are written into rows of a table, then read out by column. FIG. 3 shows an example of 100 bits, written into a 10xc3x9710 table.
Another example of interleaving is diagonal interleaving, in which data from different frames are interleaved together. Diagonal interleaving is illustrated in FIG. 4. Each frame is block interleaved using block interleavers 50a, 50b, and 50c. Using switches 52a, 52b, and 52c, interleaved bits from each frame are split into two groups. The multiplexors 54a and 54b combine groups of bits from different frames to form transmit frames. In TDMA systems, different transmit frames generally would be sent in different time slots.
Referring again to FIG. 2, the modulator 44 may apply any of a variety of modulations. Higher-order modulations, such as those illustrated in FIGS. 5A and 5B, are frequently utilized. One example is 8-PSK (eight phase shift keying), in which 3 bits are sent using one of 8 constellation points in the in-phase (I)/quadrature (Q) (or complex) plane. In FIG. 5A, 8-PSK with Gray coding is shown in which adjacent symbols differ by only one bit. Another example is 16-QAM (sixteen quadrature amplitude modulation), in which 4 bits are sent at the same time as illustrated in FIG. 5B. Higher-order modulation may be used with conventional, narrowband transmission as well as with spread-spectrum transmission.
A conventional baseband processor is shown in FIG. 6. A baseband received signal is provided to the demodulator and soft information generator 60 which produces soft bit values. These soft bit values are provided to the soft information de-interleaver 62 which reorders the soft bit values to provide de-interleaved soft bits. These de-interleaved soft bits are provided to the FEC decoder 64 which performs, for example, convolutional decoding, to produce detected information bits.
A second example of a conventional baseband processor is shown in FIG. 7. This processor employs multipass equalization, in which results, after decoding has completed, are passed back to the equalization circuit to re-equalize, and possibly re-decode, the received signal. Such a system is described, for example, in U.S. Pat. No. 5,673,291 to Dent et al. entitled xe2x80x9cSimultaneous demodulation and decoding of a digitally modulated radio signal using known symbolsxe2x80x9d which is hereby incorporated herein by reference. For the circuit illustrated in FIG. 7, the processor typically initially performs conventional equalization and decoding. After decoding, the detected information bits are re-encoded in the re-encoder 74 and then re-interleaved in the re-interleaver 72 to provide information to the multipass equalizer and soft information generator 70 which re-equalizes the received baseband signal using the detected bit values. Typically, because of diagonal interleaving or the fact that some bits are not convolutionally encoded, the second pass effectively uses error corrected bits, as determined and corrected in the first pass, to help detection of other bits, such as bits which were not error correction encoded.
Both single pass and multipass baseband processors as described above typically use conventional forward error correction (FEC) decoders. Conventional FEC decoders typically treat each soft bit value as if it were independent of all other values. For example, in a Viterbi decoder for convolutional codes, soft bit values are generally correlated to hypothetical code bit values and added. As the soft bit values typically correspond to loglikelihood values, adding soft values corresponds to adding loglikelihoods or multiplying probabilities. As the Viterbi decoder corresponds to maximum likelihood sequence estimation (MLSE) decoding, multiplying probabilities generally assumes that the noise on each bit value is independent.
For lower-order modulation, with Nyquist pulse shaping and nondispersive channels, independent noise is often a reasonable assumption. For example, for quadrature phase shift keyed (QPSK) modulation, one bit is generally sent on the I component and a second bit is sent on the Q component. Because noise is typically uncorrelated between the I and Q components, the noise on these two bits would generally be independent. However, with higher-order modulation, noise values on the different bits are generally not independent. Consider the 8-PSK example shown in FIG. 5A. As 3 bits are affected by only 2 independent noise values (I and Q noise components), the noise on the 3 soft bit values is expected to be correlated. Thus, with higher-order modulation, the conventional approaches to demodulation and decoding ignore the fact that bit soft values may be related through correlated noise. As a result, performance may be reduced.
In a more general sense, bit likelihoods may be coupled in many ways. For 8-PSK with Nyquist pulse shaping, groups of 3 bits are generally coupled by the modulation. With partial response pulse shaping, overlapping groups of bits may be coupled through the pulse shape. Differential modulation may also couple successive symbols. Bit coupling may also be introduced by the communication channel. For example, bit coupling may result from multipath time dispersion, in which symbols overlap with one another. Such overlap may result due to signal echoes.
Bit interdependence is generally present in a variety of standard protocols. For IS-136, which uses differential QPSK (DQPSK) modulation, Gray mapping is typically used to map bit pairs to so-called differential symbols, and, as a result, the two bits generally have a low interdependence. However, there is typically an interdependence between bits from adjacent symbol periods. Accordingly, multi-pass demodulation/decoding structures as described above may work well in such an environment even in non-dispersive channels.
By way of further example, the Enhanced Data Rates for Global Evolution (EDGE) system generally uses Gray mapping from triplets to 8-PSK symbols. As a result of the Gray mapping, low interdependence among bits mapping into the same symbol will typically be encountered. However, the modulator used for EDGE systems typically has a heavy partial response, and the short symbol period often makes the intersymbol interference (ISI) arising from time dispersion in the physical medium significant. As a result, a high interdependence among bits from a few neighboring symbol periods may be found.
As a further example, the Global System for Mobile communications (GSM) typically uses non-linear modulation which can be approximated by a binary linear modulator with a heavy partial response. Again, in such a system, high interdependence among bits from a few neighboring symbol periods would be expected. Thus, bit interdependence can be present in a communication system even when the mapping to symbols is chosen to lower such bit interdependence so long as information across symbols is utilized in finding bit interdependence.
One problem which may be encountered with multi-pass techniques, such as those described above, is that the re-use of the demodulator may be wasteful. For example, in many systems, such as IS-136, GSM and EDGE, the demodulator is typically an equalizer whose main task is to deal with the cumulative ISI due to the transmitter, the physical medium and the receiver. In addition, the equalizer typically conducts adjunct tasks, such as channel tracking (for example, in IS-136), noise whitening (for example, in EDGE), automatic frequency correction (AFC) and so on. Thus, the equalizer typically performs complex operations which tend to dominate the overall complexity of the baseband processor. While various of the adjunct tasks could potentially be disabled in multi-pass processing, the primary ISI task typically cannot be turned off and, thus, creates a source of waste in a second or subsequent processing pass through the equalizer. If the ISI is successfully dealt with in the first pass of the equalizer, little advantage results from addressing ISI repeatedly in subsequent passes. In addition, the necessity for storing data required to re-run the equalizer may also create problems. For example, in EDGE, the decoder typically accepts data from four bursts, thus requiring the received values for the four bursts to be stored in order to iterate between the decoder and the equalizer.
In embodiments of the present invention, methods and devices for extracting a joint probability from a maximum a posteriori (MAP) decision device are provided. Probability information associated with a first symbol and a second symbol is obtained from the MAP decision device. A joint probability of the first symbol and the second symbol is determined from the probability information associated with the first symbol and the second symbol. Methods and devices for processing a signal containing information associated with a plurality of transmitted bits using the generated joint probability information are also provided. The MAP decision device in various embodiments is a demodulator or a decoder.